Holographic optical data storage is an attractive alternative to magnetic tape, magnetic disc, and optical disc storage of digital computer data. It offers high capacity and high recording and reading data rates on storage media that can be removed from the drive, as described in Holographic Data Storage, H. J. Coufal, D. Psaltis, G. T. Sincerbox, editors, (Springer-Verlag, Berlin, 2000), incorporated herein by reference. Data to be stored is written to a photosensitive storage media by overlapping an information-bearing light beam (the signal beam) with a reference light beam. When the beams are coherent, coming for example from the same laser, standing waves in the beam's interference pattern create changes in the photosensitive material's index of refraction, thus forming a hologram. The stored data can be read out by illuminating the recorded hologram with the reference beam alone: the hologram diffracts light from the reference beam to create a copy of the original information-bearing beam. Multiple holograms can be recorded within the same volume of storage media by, for example, varying the angle of the reference beam. This is known as angular multiplexing. Many other hologram-multiplexing techniques are known in the art. The use of volumetric storage enables extremely high capacities, and the parallelism inherent in page-oriented storage offers much higher data rates that conventional serial bit-at-a-time technologies.
The information to be recorded or stored is imposed on the light beam through the use of a spatial light modulator (SLM). The SLM converts input electronic data to a two-dimensional image of bright and dark pixels, for example. Light modulated by the SLM passes through the optical system of the HDS device or drive to be recorded within the storage medium. In some instances, the SLM may modulate the phase (rather than the intensity or amplitude) of the light. Typically, a lens between the SLM and the recording medium is used to form a spatial Fourier transform of the SLM image in the region where the hologram is to be recorded in the photosensitive material of the storage medium. Subsequently, when it is desired to read the data stored in the medium, the hologram stored in the recording medium is illuminated by the reference beam to reconstruct the SLM image, which can then be detected by a photodetector such as a CCD camera. One example of an SLM suitable for holographic data storage systems can be made using ferroelectric liquid crystals (FLCs) atop a CMOS backplane, constructed similarly to the microdisplay devices described in U.S. Pat. Nos. 5,748,164 and 5,808,800, the contents of which are incorporated herein by reference. These SLMs can be fabricated by techniques that are well known in the art, for example as described in “Semiconductor manufacturing techniques for ferroelectric liquid crystal microdisplays,” by Mark Handschy in Solid State Technology volume 43, pages 151-161 (2000), incorporated herein by reference.
However, several difficulties in the implementation of a practical holographic data storage system can be traced to the design and performance of the signal-beam optical path. Also, the particular FLC SLM devices described in the abovementioned patents do not make ideal write-heads. For example, when the SLM is operated as an intensity modulator, its Fourier transform contains a bright central spot, the DC spot, that is as much as 60 dB (one million times) brighter than the surrounding light intensity. This bright spot can saturate the optical recording medium, making it difficult to record and reconstruct data with high fidelity.
It is known in the art that the Fourier-plane DC bright-spot problem can be solved by introducing into the optical system a phase mask that imposes fixed, pseudo-random optical phase variations across the wave front, as is disclosed in U.S. patent application Ser. No. 11/046,197, “Phase Masks for Use in Holographic Data Storage,” now U.S. Pat. No. 7,656,768, incorporated herein by reference. That patent application disclosed the fabrication of phase masks by a variety of techniques including relief structures in either the window or mirrors of a liquid crystal on silicon (LCOS) SLM. Also disclosed there was the implementation of an integral phase mask by use of three or more electrically selected states of the liquid crystal modulators in a liquid-crystal-on-silicon (LCOS) SLM.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.